The research of the sampling method for CT saturation for numerical busbar protection

Abstract

The busbar protection is the most important relays in plants and substations of power system. It has direct effect on the security and stability for an electric power system. The busbar protection has thus a maximum degree of security against spurious operation for external short-circuits. At the same time, it ensures that in the event of internal short-circuits a tripping signal is initiated within the very short time of less than a half-cycle. The main features for modem busbar protection are rapid speed and sensitive trip for internal faults (tripping time less than 10 ms), stability for external faults, especially during current transformer (CT) saturation. So the protection algorithm must provide simplicity, rapid computation and identification. Considering the busbar protection principle and CT saturation, the sampling differential current algorithm is the better choice for the numerical busbar protection. The main requirement that a busbar protection scheme has to fulfill is proper behavior in the case of CT saturation. Many projects for CT saturation have successfully been used in busbar protections. For example, the 'mid-impedance principle' using a very simple design to achieve a rapid, stable and sensitive detection for a fault condition, the 'checking CT saturation unit' identifying whether the CT saturating or not, are the effective methods. The 'maximum-value holding method' applied prior to the Fourier filtering is largely insensitive to CT saturation phenomena. The high-impedance busbar protection and the linear couplers can be used to overcome the problems caused by CT saturation, and so on. These methods have been used in the historical or the new products. Two methods, the synchronizing identification and the phase current comparison, are discussed in this paper.